Bracing are designed as pinned members acting in axial tension or compression only, as per the design methodology given in SCI P 358 "Joints in Steel Construction: Simple Joints to Eurocode 3". When designed as standalone connections, this is reflected in the fact that the force inputs for bracing members allow only axial forces. In the case of a bracing design linked to a MasterFrame model, the ends of the bracing member must, therefore, be pinned by the application of suitable end releases.
In all bracing connections, whether connecting to the member about the major or minor axis, the gusset plate is assumed to be aligned with the centroid of the member. Thus no additional torsion is induced in the main member due to the positioning of the brace.
In most cases, bracing connections were traditionally designed on the basis that all forces intersected with the member section lines. In this case, no additional moments would be incurred. However, the reality of forming a connection geometry can mean that such an assumption is not realistic or achievable without the use of large gusset plates which can have a detrimental effect on both the cost and structural performance of the bracing connection. To account for the forces which do not intersect, the MasterKey Bracing connections allow for displacement of bracing relative to other brace members and to the beam or column to which the brace is connected. Where forces are not acting through a common point, additional forces in the supporting members will occur. The MasterKey Bracing connections will determine the magnitude of these additional forces.
The connection between the bracing element and gusset plate is formed by a lap joint. Thus the applied force is not concentric to the gusset plate, resulting in a bending moment being generated within the gusset. The effect of the eccentricity is magnified where the use of an and L-plate end connector results in the bracing member being further displaced relative to the gusset plate.
Where a connection is in tension, buckling is not possible and so the effect of an additional moment due to the lap of the connection is ignored. Where an L-plate connection is used with a hollow section bracing member, the angle forming the L-plate is checked for bending. However, the leg of the L-plate is not free to bend since it is connected to a bracing member which is significantly stiffer in bending than the L-plate. The bending is therefore modified in the proportion of the stiffnesses of the connected elements.
For a connection in compression, the Green Book provides a method to employ a yield line analysis to determine the buckling capacity of tab plates and the gusset plate. This method applies to gusset plates connected on one or two edges. As per section 8.5 of SCI P358, this method is limited to:
•Connections made with lapped plates in the form of tab plates, T-connectors or plates notched into the bracing member
•Plates of thickness not greater than 20mm
•Bolts in square or rectangular patterns.
•Bracing angles between 20 and 160 degrees from horizontal
The design method presented in Section 8.5 of SCI P358 does not, therefore, apply to connections made directly between the bracing member and the gusset plate, such as occurs between flat plates, angles or channel sections. The Green Book method is also not applicable for connection with a row of single bolts (1 bolt per column) nor with bolts in a staggered pattern.
Where bracing connections are in compression but the above requirements for the application of the methods in SCI P358 Section 8.5 are not met, the Masterkey Connection module applies the rules from BS EN 1993-1-1 Section to design the gusset plate as a axial compression member. This is based on a determination of an effective width of an equivalent compressive strut and also an effective length factor and effective length of the compressive strut. The width of the compression strut is determined by the geometry of the bolts in the connection, while the length of the strut is based on the position of the bolts relative to the welded edge of the gusset plate.
The compression strut is based on a 30 degree dispersal angle starting from the centre of the outer bolt column and ending at a line through the centre of the inner most bolt column, as presented in earlier versions of the Green Books. Thus the width of the equivalent compressive strut is dependent upon the number and centres of bolts in both the bolt rows and bolt columns. In the case where a single bolt is specified, the effective width of the compression strut ends up being equal to the bolt diameter and is unlikely to be satisfactory except in case with a very small axial force.
In compression, it is important that the gusset plates yields first and not the welds. To ensure this is the case, SCI P 358 requires that a gusset plate welded on one edge must use full strength welds. Where the gusset plate is welded on two edges, full strength welds must be provided over a zone beyond a line taken at 5 times the gusset thickness ( 5 * tgus ) from the further point of support. Due to the difficulty of specifying the length over which the full strength weld must extend, the MasterKey: Connections module uses full strength welds over the full welded plate length in both gusset plates welded on one or two sides. To achieve a full strength weld, the software requires the weld leg to be a minimum of 0.7 * tgus
For gusset plates in tension, the forces in the gusset plate are resolved and the weld capacity is checked. In the case of a single weld, the forces are resolved to give a tension and longitudinal shear force on the weld. Where the gusset plate is welded on two sided, the forces in the brace are resolved and these forces are resisted by the welds in longitudinal shear. There is no dimensional limit on the weld sizes for bracing in tension.
For hollow sections connected to the gusset plate by means of a T-plate connector, given the relative stiffness of the T-plate connector, the distribution of tensile force local to the T-plate and hollow section connection is such that it to assume a dispersal of the tensile force over the full hollow section cross section would not account for the actual stress concentration and would potentially lead to a non-conservative design.
To account for this stress concentration, the SCI Green Book uses a 1:2.5 load dispersal through the T-plate connector to determine an effective width of section which is used to determine the tensile capacity of the section and also to determine the weld capacity. Due to the use of an effective width which reduces the effective cross section of the hollow section member local to the connection, the tensile capacity of the brace may be significantly reduced from the overall section capacity.
The use of an embedded plate with sufficient embedment such that the force in the bracing member can be dissipated over the full cross section may be required where the use of a T-plate would result in a reduction in the section tensile capacity below the applied tensile force.